Method of producing magnetic recording media

ABSTRACT

Disclosed herein is a method of producing a magnetic recording medium, wherein a magnetic coating formulation containing a hexagonal ferrite magnetic powder is applied to the surface of a substrate. It is characterized in that the hexagonal ferrite magnetic powder is represented by the following general composition formula [I] and has an average particle diameter of at most 0.1 μm , a ratio of the maximum diameter to the maximum thickness (a plate ratio) of at most 15, a particle size distribution (σgd) of at most 3.4 in terms of the geometric standard deviation based on the number and a coercive force of 200-3,000 oe, and a solvent containing at least 15 wt. % of at least one ketone compound having at most 10 carbon atoms is used in the magnetic coating formulation. 
     
         (Fe).sub.a (Co).sub.b (M.sup.1).sub.c (M.sup.2).sub.d (M.sup.3).sub.e 
    
      (O) f                                                  [I] 
     wherein M 1  means at least one metal element selected from Ba, Sr, Ca and Pb, M 2  denotes at least one metal element selected from Ti, Zr, Hf, Si, Ge, Sn, Mn, Mo, W, V, Ce, Nd, Sm, B and Nb, M 3  stands for at least one metal element selected from Mg, Ni, Cr, Cu, Zn, Cd, In, Ga, Bi, La, Y, P, Sb, and Al, a, b, c, d, e and f respectively represent the numbers of Fe, Co, M 1 , M 2 , M 3  and O atoms, a is a number of 8.0 to 12.0, b is a number of 0 to 0.5, a ratio of b to a is at most 0.055, c is a number of 0.3 to 6.0, d and e are individually a number of 0 to 6.0 with the proviso that when b is a number other than 0 and M 2  is Ti alone, e is a number other than 0, d+e is a number of 0.1 to 8.0, and f is the number of oxygen atoms satisfying the atomic valences of the other elements.

FIELD OF THE INVENTION

This invention relates to a method of producing a magnetic recordingmedium. More specifically, this invention is concerned with a method ofproducing a high-density magnetic recording medium using a hexagonalferrite magnetic powder composed of fine particles.

According to the production method of this invention, a magneticrecording medium having a coercive force controlled to a desired degreewith good precision can be produced. The method is therefore suitable,in particular, for use as a production method for high-density magneticrecording media.

BACKGROUND OF THE INVENTION

In recent years, high-density magnetic recording media obtained bycoating the surface of a substrate with a hexagonal ferrite magneticpowder having uniaxitial magnetocrystalline anisotropy have attracted agreat deal of attention.

There is an advance in development, in particular, of magnetic recordingmedia with a Co-substituted barium ferrite magnetic powder applied tothe surface of a substrate, for example, of floppy disks, hard disks andmagnetic tapes. The Co-substituted barium ferrite magnetic powder can beobtained by substituting Co for part of Fe, thereby reducing thecoercive force (in general, about 5,000 Oe) inherent in barium ferriteto a coercive force (200-3,000 Oe) suitable for use as magneticrecording media.

By the way, when industrially mass-producing magnetic recording media,the variation tolerance in coercive force of an intended magneticrecording medium is required to be within (the preset value ±20) Oe fromthe necessity of keeping the quality of various characteristics andproperties, including electromagnetic conversion characteristics,constant.

In order to control the coercive force of a magnetic recording mediumwith such good precision, the following conditions are required. Namely,(1) the coercive force of a magnetic powder used is stable and keeps aconstant value and (2) when a magnetic coating formulation containingthis magnetic powder is dispersed and coated on the surface of asubstrate to form a magnetic recording medium, a coercive force forecastis not varied.

However, it has been known that with respect to conventionally knownCo-substituted barium ferrite magnetic powders containing Co in a greatamount, their coercive forces are varied due to agglomeration caused bymagnetostatic interaction between particles of the magnetic powders[Yokoyama, et al., Journal of the Magnetics Society of Japan, Vol. 13,No. 3, pp. 488-489 (1989)].

Namely, as indicated in the simultaneously rotating model of theStoner-Wohlfarth's magnetization, ultrafine particles of the bariumferrite are susceptible to the magnetostatic interaction between theparticles of the magnetic powder because the crystalline anisotropicmagnetic field (H_(A)) and the anisotropic magnetic field (H_(K)) of themagnetic particles are decreased to a great extent by the substitutionof Co. Therefore, the coalescence and agglomeration of the magneticparticles tend to occur and the coercive force varies in a wide rangedepending upon their coalescent states.

In deed, as shown in Table 1, the conventionally known Co-substitutedbarium ferrite magnetic powders have the same composition, but theircoercive forces are different from one another in a wide range.

In addition, the present inventor was found that when Co-substitutedbarium ferrite magnetic powders are treated for the purpose of adjustingtheir water contents or improving their dispersibility, their coerciveforces vary greatly depending on the states of the magnetic powdersafter treatment even in those of the same preparation lot (seeComparative Example 1 which will be described subsequently).

As apparent from these facts, the coercive forces of the knownCo-substituted barium ferrite magnetic powders proposed to use asmagnetic recording media and containing a relatively great amount of Covary depending on their coalescent states and the like. It is thereforeimpossible to satisfy the above-mentioned requirement that the coerciveforce is stable and keeps a constant value.

                  TABLE 1                                                         ______________________________________                                                             Magnetic properties                                              Example  Barium ferrite                                                                              Hc     σs                                JPO     No.      composition   (Oe)   (emu/g)                                 ______________________________________                                        149328/81                                                                             Example  Ba.sub.1.2 Fe.sub.10.4 Co.sub.0.8 Ti.sub.0.8                                                870    47                                              1                                                                     60002/81                                                                              Example  Ba.sub.1.2 Fe.sub.10.4 Co.sub.0.8 Ti.sub.0.8                                                950    58                                      56326/82                                                                              Example  Ba.sub.1.2 Fe.sub.10.4 Co.sub.0.8 Ti.sub.0.8                                                1300   60                                      95902/85                                                                              Example  Ba.sub.1.2 Fe.sub.10.4 Co.sub.0.8 Ti.sub.0.8                                                680    39                                              2                                                                     ______________________________________                                         Note:                                                                         JPO stands for Japanese Patent Application LaidOpen No.                  

In addition, magnetic recording media making separate use of theconventionally known Co-containing hexagonal barium ferrite magneticpowders are accompanied by a problem that the coercive forces of themagnetic powders used vary greatly from those of the magnetic recordingmedia formed therefrom.

For example, as shown in Table 2, the coercive forces of the magneticrecording media increase to about at least 1.1 times, and sometimes,about 2.0 times the coercive forces of their corresponding Co-containinghexagonal barium ferrite magnetic powders used and moreover, since thedegree of variation in coercive force differs with compositions, it isimpossible to accurately forecast a relation between the coercive forceof each of the magnetic powders and the coercive force of the magneticrecording medium produced therefrom. It is hence very difficult tosatisfy the above-mentioned requirement that when a magnetic coatingformulation containing a magnetic powder is dispersed and coated on thesurface of a substrate to form a magnetic recording medium, a coerciveforce forecast is not varied.

For this reason, it is difficult to industrially mass-produce magneticrecording media having a fixed quality from any Co-containing bariumferrite magnetic powders by any conventional methods.

                                      TABLE 2                                     __________________________________________________________________________          Co-containing hexagonal                                                                     Coercive force                                                                         Coercive force of                                                                       Hc of Tape/                                  barium ferrite magnetic                                                                     of magnetic                                                                            magnetic recording                                                                      Hc of magnetic                         JPO   powder        powder, Hc (Oe)                                                                        medium, Hc (Oe)                                                                         powder                                 __________________________________________________________________________    157718/85                                                                           Ba.sub.1.0 Fe.sub.8.0 Co.sub.0.40                                                           1110     1210      1.09                                   168532/86                                                                           Ba.sub.1.0 Fe.sub.10.4 Co.sub.0.80 Ti.sub.0.80                                              845      925       1.09                                   207720/87                                                                           Ba.sub.1.0 Fe.sub.5.73 Co.sub.0.42 Ti.sub.0.42 Si.sub.0.1                                   515      652       1.27                                   216922/87                                                                           Ba.sub.1.0 Fe.sub.10.2 Co.sub.0.9 Ti.sub.0.45 Sn.sub.0.45                                   550      800       1.45                                   235220/87                                                                           Ba.sub.1.0 Fe.sub.6.93 Co.sub.0.53 Ti.sub.0.53                                              1310     1450      1.11                                    64626/88                                                                           Co-substituted barium ferrite                                                               550      620       1.13                                   __________________________________________________________________________     Note:                                                                         JPO stands for Japanese Patent Application LaidOpen No.                  

Further, the above-cited literature [Yokoyama, et al., Journal of theMagnetics Society of Japan, Vol. 13, No. 3, pp. 488-489(1989)]suggeststhat the coercive force of barium ferrite magnetic powder variesdepending upon the coalescent state (dispersibility) of the magneticpowder even when using any substituting elements without limiting to Coso long as they are elements capable of reducing the coercive force ofthe magnetic powder. In other words, it suggests that any barium ferritemagnetic powders having a coercive force reduced to a value suitable foruse as magnetic recording media are varied in coercive force dependingupon their coalescent states.

Accordingly, it is very difficult to industrially mass-produce magneticrecording media having the same quality from any substituted hexagonalferrite magnetic powders, whose coercive forces have been reduced, bythe conventional methods.

OBJECT AND SUMMARY OF THE INVENTION

It is an object of this invention is to provide a production method of amagnetic recording medium in which when a hexagonal ferrite magneticpowder, whose coercive force has been reduced to a level suitable foruse in magnetic recording, is dispersed and coated on the surface of asubstrate to form the magnetic recording medium, the coercive forcevaries only a little.

The present inventor has carried out an extensive investigation with aview toward attaining this object. As a result, it has surprisingly beenfound that (a) hexagonal ferrite magnetic powders containing specificelements within specified composition ranges and having the specifiedranges of properties as to magnetic powder, of all magnetic powders, isvaried in coercive force only to such a little degree that theirvariations in coercive force substantially pose no problem, underordinary conditions and (b) when each of these magnetic powders is usedtogether with a solvent containing at least one specified organiccompound to prepare a magnetic coating formulation and the magneticcoating formulation is applied to the surface of a substrate into amagnetic recording medium, a great variation does not occur between thecoercive force of the magnetic powder and that of the magnetic recordingmedium produced. Moreover, it has been found that even when thehexagonal ferrite magnetic powder contains Co as a substituting element,the coercive force of the resulting magnetic recording medium ca becontrolled with good precision.

The present invention has been led to completion on the basis of thesefindings.

According to this invention, there is thus provided a method ofproducing a magnetic recording medium in which a magnetic coatingformulation containing a hexagonal ferrite magnetic powder is applied tothe surface of a substrate, characterized in that the hexagonal ferritemagnetic powder is represented by the following general compositionformula [I]and has an average particle diameter of at most 0.1 μm, aratio of the maximum diameter to the maximum thickness (a plate ratio)of at most 15, a particle size distribution (σgd) of at most 3.4 interms of the geometric standard deviation based on the number and acoercive force of 200-3,000 Oe, and a solvent containing at least 15 wt.% of at least one ketone compound having at most 10 carbon atoms is usedin the magnetic coating formulation.

    (Fe).sub.a (Co).sub.b (M.sup.1).sub.c (M.sup.2).sub.d (M.sup.3).sub.e (0).sub.f                                                 [I]

wherein M¹ means at least one metal element selected from Ba, Sr, Ca andPb, M² denotes at least one metal element selected from Ti, Zr, Hf, Si,Ge, Sn, Mn, Mo, W, V, Ce, Nd, Sm, B and Nb, M³ stands for at least onemetal element selected from Mg, Ni, Cr, Cu, Zn, Cd, In, Ga, Bi, La, Y,P, Sb and Al, a, b, c, d, e and f respectively represent the numbers ofFe, Co, M¹, M², M³ and 0 atoms, a is a number of 8.0 to 12.0, b is anumber of 0 to 0.5, a ratio of b to a is at most 0.055, c is a number of0.3 to 6.0, d and e are individually a number of 0 to 6.0 with theproviso that when b is a number other than 0 and M² is Ti alone, e is anumber other than 0, d +e is a number of 0.1 to 8.0, and f is the numberof oxygen atoms satisfying the atomic valences of the other elements.

The most feature of the magnetic recording medium produced in accordancewith this invention resides in that there is little variation incoercive force between the magnetic powder used and the resultingmagnetic recording medium. It is therefore possible to easily produce amagnetic recording medium having a fixed quality controlled within avariation tolerance in coercive force of (the preset value ±20) Oe bymerely using a magnetic powder having substantially the same coerciveforce as the preset value.

The magnetic powders useful in the practice of this invention are variedin coercive forces only to such a little degree that their variations incoercive force substantially pose no problem, under ordinary conditionsif their coalescent states change. It is hence possible to choose amagnetic powder suitable for an intended magnetic recording medium.

DETAILED DESCRIPTION OF THE INVENTION

Features of the present invention will hereinafter be described indetail.

Magnetic powder

The magnetic powder useful in the practice of this invention is amulti-element hexagonal ferrite having the composition represented bythe above described general composition formula [I].

It is critical in this invention that the numbers a to f of theindividual constituent elements of the magnetic powder should be withinthe above-specific ranges. If these numbers are outside the specificranges, it is difficult to produce magnetic recording mediasubstantially free of any variations in coercive force, which areintended in this invention.

Preferred proportions of the individual components are such that a is8.0 to 12.0, b is a number of 0 to 0.4, a ratio of b to a is at most0.05, c is a number of 0.3 to 4.0, d is a number of 0 to 4.0, e is anumber of 0 to 4.0 with the proviso that when b is a number other than 0and M² is Ti alone, e is a number other than 0, d +e is a number of 0.2to 5.0, and f is the number of oxygen atoms satisfying the atomicvalences of the other elements in the general composition formula [I].

If the magnetic powder used in this invention contains Co, the object ofthis invention can be attained so long as it contains the constituentelements and falls within the composition range, both, specified in thegeneral composition formula [I]. If Co is present in a compositionalproportion higher than that specified in the present invention, it isimpossible to attain the object of this invention, that is to say, thecoercive force of a magnetic powder is stable and keeps a constantvalue, and when the magnetic powder is dispersed and coated on thesurface of a substrate as a magnetic coating formulation to form amagnetic recording medium, a coercive force forecast is not varied.

Moreover, with respect to the magnetic powders useful in the practice ofthis invention, the properties as to powder are important.

First of all, the average particle diameter of the maximum diameters ofhexagonal plate crystals must be at most 0.1 μm. If the average particlediameter of the magnetic powder exceeds 0.1 μm, its coercive force isstable, but the variation in coercive force becomes a nonnegligibledegree when a magnetic recording medium is produced therefrom inaccordance with the method of this invention.

The plate ratio (a quotient obtained by dividing the maximum diameter ofthe hexagonal plate crystal by its maximum thickness) must be at most15, preferably at most 10. If the plate ratio exceeds 15, the magneticpowder is very thin and hence tends to agglomerate, so that bothcoercive forces of the magnetic powder and magnetic recording mediumproduced therefrom become more unstable as the degree of theagglomeration increases.

The particle size distribution (σgd) in terms of the geometric standarddeviation based on the number must be at most 3.4, preferably at most3.1. If σgd exceeds 3.4 and the particle size distribution is hencewide, the magnetic powder tends to agglomerate and at the same time, thecoercive force distribution of the magnetic powder also becomes wider.As a result, the coercive force of the magnetic powder tends to becomeunstable and moreover, the coercive force of the magnetic recordingmedium produced therefrom also tends to vary and hence becomes unstable.

By the way, these properties of the powder are obtained by statisticallyarranging the measurements obtained about at least 400 particles of eachmagnetic powder, which have been sampled at random. With respect to theshape of the magnetic powder on the other hand, it does not necessarilyexhibit the form of a regular hexagonal plate in some cases, dependingupon its composition, production conditions and/or the like. However, noparticular problem arises so long as it falls within the scope of themagnetic powder defined in this invention.

The coercive force of the magnetic powder must be controlled within arange of 200-3,000 Oe, which permits magnetic recording.

No particular limitation is imposed on the production method of thehexagonal ferrite magnetic powder useful in the practice of thisinvention, and any suitable production method may be used. As exemplaryproduction methods, may be mentioned a coprecipitation method, fluxmethod, coprecipitation-flux method, hydrothermal synthesis method,glass crystallization method and the like.

Magnetic coating formulation

The magnetic coating formulation is generally prepared by dispersing andkneading the magnetic powder together with a dispersant, solvent, binderand one or more other additives.

In this invention, the magnetic coating formulation can be prepared by amethod known per se in the art except that the method requires that themagnetic powder is a specific hexagonal ferrite magnetic powderdescribed above and the solvent contains at least 15 wt. % of at leastone ketone compound having at most 10 carbon atoms.

In the magnetic coating formulation, there has heretofore been used, asa single solvent or mixed solvent, for example, ketones such as acetone,methyl ethyl ketone, methyl isobutyl ketone and cyclohexanone; esterssuch as ethyl formate, ethyl acetate and butyl acetate; alcohols such asmethanol, ethanol, isopropanol and butanol; aromatic hydrocarbons suchas toluene, xylene and ethylbenzene; ethers such as isopropyl ether,ethyl ether and dioxane; furans such as tetrahydorfuran and furfural;and the like.

However, the solvent useful in the practice of this invention mustcontain at least 15 wt. % of at least one ketone compound (hereinaftermay referred to as a "ketone") having at most 10 carbon atoms. If thesolvent used contains no ketone compound or, if contained, its amount isless than 15 wt. %, the coercive force of a resulting magnetic recordingmedium becomes unstable and it is hence difficult to forecast thecoercive force of the resulting magnetic recording medium from thecoercive force of the magnetic powder. A preferred amount of theketone(s) contained in the solvent is 15-90 wt. %.

As exemplary ketones used in this invention and having at most 10 carbonatoms, may be mentioned acetone, methyl ethyl ketone, methyl isobutylketone, cyclohexanone, methyl n-propyl ketone, diethyl ketone,hexanone-2, hexanone-3, di-n-propyl ketone, diisopropyl ketone,diisobutyl ketone, di-n-amyl ketone, acetylacetone, phorone,cyclopentanone, cycloheptanone and the like.

As the binder, may be suitably used various resins such as vinylchloride-vinyl acetate copolymers, polyurethane resins, phenol resinsand epoxy resins.

As the dispersant, may be used a wide variety of dispersants such asvarious fatty acids and lecithins.

It is necessary to thoroughly disperse the magnetic powder by means of adispersing machine such as a ball mill, sand mill or triple roll millupon the preparation of the magnetic coating formulation.

The production of a magnetic recording medium may be conducted inaccordance with a method known per se in the art. For example, it isonly necessary to add suitably a hardening agent such as an isocyanateto the magnetic coating formulation, apply the resulting mixture by acoater such as a reverse-roll coater, gravure coater or spin coater to asubstrate, for example, a polyethylene terephthalate film, subject theresulting coating film to an orientation treatment by a magnetic fieldin a horizontal or perpendicular direction as needed, dry thethus-treated substrate, subject it to a surface-forming treatment, cureand age the thus-treated substrate, polish it as needed and then cut itinto desired pieces. It is of course possible to provide a back coatand/or a top coat, if necessary.

ADVANTAGES OF THE INVENTION

Since the variation in coercive force of the magnetic recording mediumobtained in accordance with the method of the present invention from themagnetic powder used is extremely small, it is possible to substantiallycontrol the variation tolerance in coercive force of an intendedmagnetic recording medium within (the preset value ±20 Oe) by suitablychoosing the coercive force of the magnetic powder used according to thedesired coercive force of the magnetic recording medium.

The fact that the coercive force of the magnetic recording medium can becontrolled with good precision, as described above, is a surprisingadvantage quite unpredictable from the findings and suggestions from theconventional literature.

Owing to this invention, it has been possible to industriallymass-produce magnetic recording media having the same quality using thehexagonal ferrite magnetic powders.

EMBODIMENTS OF THE INVENTION

The present invention will hereinafter be described more specifically bythe following Examples and Comparative Examples. However, it should beborne in mind that this invention is not limited to the followingExamples only. Incidentally, all designations of "part" or "parts" and"%" as will be used in the Examples and Comparative Examples mean partor parts by weight and wt. % unless otherwise provided.

Preparation of Magnetic Powders

Magnetic powders having their corresponding compositions shown in Table3 were prepared in accordance with the coprecipitation-flux method.

In each of the magnetic powders, its water content is 0.6% (supposingthe water content where the magnetic powder is vacuum-dried at 150° C.for 20 hours is 0%). By the way, in the composition formulae of themagnetic powders in Table 3, the atomic ratios of the elements wereexpressed by those of the elements at the time og preparing the startingmaterials, and the indication of oxygen was omitted for simplification.

Production of Magnetic Recording Media

Magnetic recording media described in the following Examples andComparative Examples were produced by the following process.

Namely, a sample magnetic powder in an amount of 100 parts by weight wasmixed thoroughly with 10 parts of a binder for magnetic tape ("MR-110",trade name; product of Nippon Zeon Co., Ltd.), 10 parts of apolyurethane resin, 3 parts of conductive carbon black, 5 parts ofhighly purified alumina powder, 1.2 parts of additives such as stearicacid and 150 parts of one of the following various solvents to prepare amagnetic coating formulation. The magnetic coating formulation wasapplied by a roll coater to a polyethylene terephthalate film withoutany orientation. The thus-coated polyethylene terephthalate film wasdried and then caused to pass through between calender rolls.Thereafter, the thus-treated film was slitted into strips of 1/2 inchwide, thereby obtaining magnetic tapes.

    ______________________________________                                        Solvents for the magnetic coating formulations:                               ______________________________________                                        Solvent 1: (Ketone compound content = 85%)                                    Methyl ethyl ketone       40%                                                 Cyclohexanone             45%                                                 Toluene                   15%                                                 Solvent 2: (Ketone compound content = 50%)                                    Methyl isobutyl ketone    10%                                                 Cyclohexanone             40%                                                 Toluene                   40%                                                 Tetrahydrofuran           10%                                                 Solvent 3: (Ketone compound content = 20%)                                    Methyl ethyl ketone        5%                                                 Cyclohexanone             15%                                                 Toluene                   60%                                                 Tetrahydrofuran           20%                                                 Solvent 4: (Ketone compound content = 12%)                                    Methyl ethyl ketone        6%                                                 Cyclohexanone              6%                                                 Toluene                   50%                                                 Tetrahydrofuran           38%                                                 Solvent 5: (Ketone compound content = 10%)                                    Methyl isobutyl ketone     2%                                                 Cyclohexanone              8%                                                 Toluene                   50%                                                 Tetrahydrofuran           40%                                                 ______________________________________                                    

Measurements of Physical Properties and Others

Various physical properties and characteristics in the Examples andComparative Examples were measured in accordance with the followingmethods.

Coercive Force (Hc) and Saturation Magnetization (σs) of Magnetic Powder

The coercive forces and saturation magnetizations in these examples weremeasured by using a vibrating sample magnetometer (VSM) at a maximumapplied magnetic field of 10 KOe and a measuring temperature of 28° C.

Average Particle Diameter (D) and Plate Ratio (D/L):

The maximum diameters and maximum thicknesses of at least 400 particleswere measured from a micrograph of the magnetic powder taken through atransmission-type electron microscope to calculate the average particlediameter and the ratio of the maximum diameter to the maximum thickness(the plate ratio) as arithmetic averages.

Particle Size Distribution (σgd):

According to the above measurements of the average particle diameter andplate ratio, the particle size distribution (σgd) in terms of thegeometric standard deviation based on the number was determined. Thecalculation formula of σgd is as follows: ##EQU1## Both d₅₀ and du₈₄ 13were determined from a logarithmic normal distribution histogram.

Coercive Force (Hc⊥) of Magnetic Tape

The coercive force in a perpendicular direction of each magnetic tapewas measured by using the vibrating sample magnetometer (VSM) at amaximum applied magnetic field of 5 KOe (a measuring temperature: 28°C.).

                                      TABLE 3                                     __________________________________________________________________________           Composition of   Properties of magnetic powder                         Magnetic                                                                             magnetic powder  Hc  σs                                                                           D                                            powder No.                                                                           (atomic ratio)   (Oe)                                                                              (emu/g)                                                                            ( Å )                                                                         D/L                                                                              σgd                             __________________________________________________________________________     (1)   Ba.sub.1.1 Fe.sub.10.9 Zr.sub.0.7 Ni.sub.0.7 W.sub.0.4                                         596 57   720 7.9                                                                              2.8                                    (2)   Ba.sub.1.1 Fe.sub.10.6 Zr.sub.0.6 Cu.sub.0.8 Si.sub.0.5                                        734 56   630 3.8                                                                              2.4                                    (3)   Ba.sub.1.5 Fe.sub.11.0 Ti.sub.0.5 Si.sub.2.0 Ni.sub.0.5                                        1250                                                                              58   610 4.3                                                                              2.3                                    (4)   Ba.sub.1.1 Fe.sub.10.2 Sn.sub.0.9 Ni.sub.0.9                                                   752 55   540 2.3                                                                              2.1                                    (5)   Ba.sub.1.9 Fe.sub.11.2 Sn.sub.0.4 Zn.sub.0.4 La.sub.0.1 Si.sub.1.4                             1440                                                                              57   590 2.9                                                                              2.4                                    (6)   Ba.sub.1.2 Fe.sub.10.6 Zr.sub.0.5 Cu.sub.0.7 Ti.sub.0.2 Si.sub.0.5                             741 54   530 3.3                                                                              2.4                                    (7)   Ba.sub.1.1 Fe.sub.10.2 Zr.sub.0.9 Cr.sub.0.9                                                   1680                                                                              53   880 6.3                                                                              2.9                                    (8)   Ba.sub.1.1 Fe.sub.10.0 Ti.sub.1.1 Zn.sub.1.1                                                   415 57   570 3.1                                                                              2.3                                    (9)   Ba.sub.0.9 Fe.sub.10.5 Ti.sub.1.5 Cu.sub.0.5                                                   1530                                                                              54   750 4.7                                                                              2.6                                   (10)   Ba.sub.1.1 Fe.sub.11.3 In.sub.0.7 Si.sub.3.0                                                   882 51   730 5.5                                                                              2.8                                   (11)   Ba.sub.1.1 Fe.sub.11.0 Zr.sub.0.5 Mg.sub.0.5 Si.sub.0.7                                        947 55   640 4.1                                                                              2.3                                   (12)   Ba.sub.1.1 Fe.sub.10.4 Si.sub.1.6 Ni.sub.0.8                                                   2760                                                                              57   810 5.6                                                                              2.4                                   (13)   Ba.sub.0.8 Fe.sub.10.2 Zr.sub.0.6 Zn.sub.0.85                                                  649 55   700 3.5                                                                              2.7                                   (14)   Ba.sub.1.1 Fe.sub.11.4 Si.sub.0.5 La.sub.0.6                                                   2560                                                                              59   590 3.0                                                                              2.2                                   (15)   Ba.sub.1.1 Fe.sub.10.6 Ti.sub.0.7 Mg.sub.0.7 Si.sub.0.5                                        845 55   620 4.4                                                                              2.6                                   (16)   Ba.sub.2.5 Fe.sub.10.8 Zr.sub.0.6 Si.sub.2.5 Zn.sub.0.4 Ni.sub.0.2                             713 55   570 3.7                                                                              2.4                                   (17)   Ba.sub.1.1 Fe.sub.10.8 Sn.sub.0.6 Mg.sub.0.6 Si.sub.1.8                                        1050                                                                              54   680 4.8                                                                              2.3                                   (18)   Ba.sub.1.1 Fe.sub.11.0 Ti.sub.1.5 Zr.sub.0.5 Cu.sub.0.5 Si.sub.0.7                             754 55   600 3.4                                                                              2.3                                   (19)   Ba.sub.1.1 Fe.sub.10.4 Sn.sub.0.8 Mg.sub.0.8 Ti.sub.0.6 Si.sub.0.3                             311 52   610 3.1                                                                              2.4                                   (20)   Ba.sub.1.1 Fe.sub.11.0 Sn.sub.0.9 Al.sub.4.0 Cu.sub.0.9                                        358 53   730 3.4                                                                              2.4                                   (21)   Ba.sub.1.1 Fe.sub.10.7 Sn.sub.0.65 Si.sub.0.5 Cu.sub.0.65 W.sub.0.1           1                725 54   640 3.2                                                                              2.5                                   (22)   Ba.sub.1.5 Fe.sub.11.1 Nb.sub.0.3 Zn.sub.0.6 Si.sub.1.0                                        833 55   670 3.9                                                                              2.6                                   (23)   Ba.sub.0.85 Fe.sub.10.4 Sn.sub.0.6 Cr.sub.0.5 Zn.sub.0.3                                       692 54   720 3.5                                                                              2.3                                   (24)   Ba.sub.1.1 Fe.sub.11.5 Ti.sub.1.0 In.sub.0.5                                                   1230                                                                              54   770 5.1                                                                              2.8                                   (25)   Ba.sub.1.1 Fe.sub.10.4 Zr.sub.0.7 Ni.sub.0.5 Si.sub.0.5 Ti.sub.0.5                             947 55   610 4.7                                                                              2.4                                   (26)   Ba.sub.1.1 Fe.sub.10.2 Sn.sub.0.9 Ti.sub.0.9 Nd.sub.0.1                                        1680                                                                              56   790 5.6                                                                              3.0                                   (27)   Ba.sub.1.1 Fe.sub.11.2 Ni.sub.0.4 Zn.sub.0.4 Sb.sub.0.4                                        596 56   540 4.0                                                                              2.5                                   (28)   Ba.sub.1.1 Fe.sub.11.1 Zr.sub.0.8 Sn.sub.0.1 Si.sub.0.5                                        2140                                                                              54   750 4.1                                                                              2.3                                   (29)   Sr.sub.1.1 Fe.sub.11.6 Sn.sub.0.4 Mg.sub.0.4                                                   1430                                                                              57   890 3.8                                                                              2.8                                   (30)   Ba.sub.0.8 Sr.sub.0.4 Fe.sub.10.8 Zr.sub.0.6 Cu.sub.0.6                                        858 55   740 4.4                                                                              2.7                                   (31)   Ba.sub.1.0 Pb.sub.0.5 Fe.sub.10.4 Si.sub.1.2 Sn.sub.0.8 Ni.sub.0.8                             562 55   680 4.7                                                                              2.5                                   (32)   Pb.sub.1.1 Fe.sub.10.0 Sn.sub.1.0 Mg.sub.1.0 Si.sub.3.0                                        354 54   630 4.1                                                                              2.5                                   (33)   Ba.sub.0.9 Ca.sub.0.3 Fe.sub.10.2 Zr.sub.0.9 Ni.sub.0.2 Cu.sub.0.7                             489 54   770 5.6                                                                              2.6                                   (34)   Ba.sub.1.1 Fe.sub.9.6 Ce.sub.0.1 Zr.sub.1.2 Si.sub.1.5 Cu.sub.1.2                              677 54   610 4.1                                                                              2.3                                   (35)   Ba.sub.1.1 Fe.sub.11.0 Nb.sub.0.2 Ti.sub.0.4 Zn.sub.0.8 Bi.sub.0.05                            862 55   580 3.6                                                                              2.4                                   (36)   Ba.sub.1.1 Fe.sub.11.3 W.sub.0.1 Sn.sub.0.3 Ni.sub.0.3                                         2130                                                                              55   720 3.3                                                                              2.4                                   (37)   Ba.sub.2.5 Fe.sub.11.2 Sn.sub.0.4 Ge.sub.2.0 Mg.sub.0.4 Ga.sub.0.1                             1540                                                                              56   630 5.0                                                                              2.8                                   (38)   Ba.sub.1.1 Fe.sub.10.6 Sn.sub.0.7 Mo.sub.0.1 Zn.sub.0.7 Si.sub.0.8                             981 54   740 4.3                                                                              2.3                                   (39)   Ba.sub.1.1 Fe.sub.10.2 Ti.sub.0.9 Si.sub.0.5 Mn.sub.0.1 Ni.sub.0.4            Zn.sub.0.4       656 55   690 4.6                                                                              2.2                                   (40)   Ba.sub.1.6 Fe.sub.10.1 V.sub.0.6 Mn.sub.0.2 Cd.sub.0.8                                         2810                                                                              57   890 9.4                                                                              3.1                                   (41)   Ba.sub.1.1 Fe.sub.10.6 Zr.sub.0.7 Si.sub.1.5 Sb.sub.0.3 Cu.sub.0.4                             728 55   670 5.1                                                                              2.5                                   (42)   Ba.sub.1.1 Fe.sub.10.9 Hf.sub.0.4 Ti.sub.0.3 Ni.sub.0.75 Si.sub.0.9           8                833 55   710 4.1                                                                              2.4                                   (43)   Ba.sub.1.1 Fe.sub.11.0 Co.sub.0.3 Sn.sub.0.5 Mg.sub.0.2                                        544 52   680 6.8                                                                              2.0                                   (44)   Ba.sub.1.1 Fe.sub.10.0 Co.sub.0.2 W.sub.1.5 Zn.sub.0.4                                         618 53   550 4.4                                                                              1.9                                   (45)   Ba.sub.1.3 Fe.sub.11.0 Co.sub.0.1 Zr.sub.0.5 Ti.sub.                          0.5 Si.sub.0.5   955 54   590 3.1                                                                              2.1                                   (46)   Ba.sub.1.1 Fe.sub.10.8 Co.sub.0.4 V.sub.0.6 La.sub.0.2                                         713 53   600 3.8                                                                              2.3                                   (47)   Ba.sub.1.1 Fe.sub.10.1 Co.sub.0.1 Ti.sub.0.1 Si.sub.0.8 Sn.sub.0.4            Ni.sub.0.4       1033                                                                              51   650 3.8                                                                              1.7                                          Cu.sub.0.75 Sb.sub.0.3                                                 (48)   Ba.sub.1.5 Fe.sub.10.5 Co.sub.0.05 Ti.sub.0.05 Si.sub.0.3 Zr.sub.0.           7 Cu.sub.0.7     710 55   580 3.4                                                                              2.9                                   (49)   Ba.sub.1.1 Fe.sub.11.0 Co.sub.0.5 Nb.sub.0.3 Ga.sub.0.2                                        836 57   730 3.5                                                                              1.6                                   (50)   Ba.sub.1.1 Fe.sub.11.0 Co.sub.0.1 Mn.sub.0.1 Sn.sub.0.8 Sb.sub.0.5                             311 50   510 2.9                                                                              1.5                                   (51)   Ba.sub.1.1 Fe.sub.11.5 Co.sub.0.4 Ce.sub.0.1 Cr.sub.0.6                                        1130                                                                              52   690 3.6                                                                              2.3                                   (52)   Ba.sub.1.8 Fe.sub.10.4 Co.sub.0.2 Ti.sub.0.8 Zr.sub.0.6                                        449 50   620 3.1                                                                              2.8                                   (53)   Ba.sub.1.1 Fe.sub.11.0 Co.sub.0.4 Nd.sub.0.4 Y.sub.0.2                                         1560                                                                              56   630 3.5                                                                              1.8                                   (54)   Ba.sub.1.7 Fe.sub.11.2 Co.sub.0.4 Zr.sub.0.4 Cu.sub.0.4                                        748 53   610 4.7                                                                              1.7                                   (55)   Ba.sub.0.8 Pb.sub.0.4 Fe.sub.11.0 Co.sub.0.1 Ti.sub.0.1 Si.sub.4.0            Zr.sub.0.3       956 52   670 4.4                                                                              2.4                                          Mg.sub.0.2 Zn.sub.0.1                                                  (56)   Ba.sub.1.8 Fe.sub.10.8 Co.sub.0.3 Ti.sub.2.0 Ni.sub.0.1                                        2380                                                                              59   710 4.9                                                                              2.7                                   (57)   Ba.sub.1.1 Fe.sub.11.0 Co.sub.0.3 Mo.sub.0.2 Al.sub.2.0                                        2530                                                                              57   440 2.7                                                                              2.1                                   (58)   Ba.sub.1.1 Fe.sub.11.0 Co.sub.0.2 Mg.sub.0.4 Sb.sub.0.6                                        845 56   570 2.6                                                                              2.1                                   (59)   Pb.sub.1.1 Fe.sub.10.4 Co.sub.0.4 B.sub.2.0 Mg.sub.0.4                                         937 54   640 4.0                                                                              2.3                                   (60)   Ba.sub.1.0 Ca.sub.0.3 Fe.sub.10.6 Co.sub.0.2 Si.sub.1.5 Zr.sub.0.7            Zn.sub.0.5       648 56   590 3.2                                                                              2.8                                   (61)   Ca.sub.1.1 Fe.sub.11.4 Co.sub.0.3 Sm.sub.0.2 Bi.sub.0.1                                        1944                                                                              56   550 3.2                                                                              2.4                                   (62)   Ba.sub.3.0 Fe.sub.10.6 Co.sub.0.2 Si.sub.4.0 Sb.sub.0.5                                        462 53   730 4.4                                                                              3.0                                   (63)   Ba.sub.1.1 Fe.sub.10.8 Co.sub.0.1 Ti.sub.0.1 Si.sub.1.8 Sn.sub.0.5            Mg.sub.0.5       719 50   680 3.1                                                                              1.9                                   (64)   Ba.sub.1.1 Fe.sub.10.8 Co.sub.0.1 Ti.sub.0.1 Ni.sub.0.5 Sb.sub.0.5                             741 52   610 3.3                                                                              2.2                                   (65)   Ba.sub.1.1 Fe.sub.11.0 Co.sub.0.2 Ge.sub.0.8 P.sub.0.03                                        563 52   860 3.0                                                                              3.1                                   (66)   Sr.sub.1.1 Fe.sub.10.4 Co.sub.0.3 Hf.sub.0.8 Cd.sub.0.5                                        750 51   710 3.2                                                                              2.1                                   (67)   Ba.sub.1.1 Fe.sub.11.3 Co.sub.0.1 Mn.sub.0.1 In.sub.0.5                                        975 48   780 5.2                                                                              1.9                                   (68)   Ba.sub.0.8 Fe.sub.11.0 Co.sub.0.2 Ni.sub.0.4 In.sub.0.2                                        1348                                                                              53   630 4.1                                                                              2.0                                   (69)   Ba.sub.1.1 Sr.sub.1.0 Fe.sub.10.2 Co.sub.0.4 W.sub.1.0 Zn.sub.0.4                              1260                                                                              52   890 6.8                                                                              2.8                                   __________________________________________________________________________

Stability on Coercive Forces of Magnetic Powders Example 1

With respect to the magnetic powders of Magnetic Powder Nos. (2), (3),(8), (11), (17), (36), (43), (45), (47), (54) and (56) shown in Table 3,the following treatments were performed to determine the influence ofsuch treatments on the coercive forces of the magnetic powders. Resultsare shown in Table 4.

Treatment of Magnetic Powder

(a) magnetic powder was sufficiently dried to control its water contentto 0.1%.

(b) The water content of magnetic powder was controlled to 0.4%.

(c) The water content of magnetic powder was controlled to 1.0%.

(d) Magnetic powder in an amount of 2 wt. % was applied to the surfaceof SiO₂ having a diameter of 1 μm.

Comparative Example 1

A Co-containing barium ferrite magnetic powder [Magnetic powder No.(C-1)]represented by the following composition formula was prepared bythe coprecipitation-flux method.

    Ba.sub.1.1 Fe.sub.10.3 Co.sub.0.85 Ti.sub.0.85

The magnetic powder had a coercive force, Hc of 583 Oe, a saturationmagnetization, σs of 57 emu/g, an average particle diameter, D of 560 Å,a plate ratio, D/L of 3.8 and a particle size distribution, σgd of 2.8.

Regarding the thus-prepared Co-containing barium ferrite magneticpowder, the treatments of (a)-(d) were conducted in the same manner asin Example 1 to determine the influence on its coercive force. Resultsare also shown in Table 4.

It is understood from the results of Example 1 and Comparative Example 1that the variations in coercive force of the magnetic powders useful inthe practice of this invention are little even after the treatment of(a)-(d) and their coercive forces are hence stable, whereby they aresuitable for use in the production of magnetic recording media.

On the other hand, it is appreciated that the coercive force of theconventional Co-substituted barium ferrite magnetic powder prepared inComparative Example 1 and containing Co in a great amount varies to asignificant extent even upon slight changes of the water content of themagnetic powder and moreover even when the magnetic powder is applied tothe surface of SiO₂ to enhance its dispersibility, and hence is veryunstable.

Incidentally, portions of the magnetic powders useful in the practice ofthis invention are exemplified in Table 4. However, the coercive forcesof all the magnetic powders within the scope of the present inventionare as stable as the examples in Table 4 and there is substantially novariation in coercive force even after the above treatments.

                                      TABLE 4                                     __________________________________________________________________________    Coercive force, Hc (oe) of magnetic powder                                    Magnetic                                                                             Example 1                        Comp. Ex. 1                           powder No.                                                                           (2)                                                                              (3)                                                                              (8)                                                                              (11)                                                                             (17)                                                                             (36)                                                                             (43)                                                                             (45)                                                                             (47)                                                                             (54)                                                                             (56)                                                                             (C-1)                                 __________________________________________________________________________    Untreated                                                                            734                                                                              1250                                                                             415                                                                              947                                                                              1050                                                                             2130                                                                             544                                                                              955                                                                              1033                                                                             748                                                                              2380                                                                             583                                   Treatment (a)                                                                        731                                                                              1254                                                                             415                                                                              945                                                                              1050                                                                             2129                                                                             541                                                                              950                                                                              1034                                                                             748                                                                              2385                                                                             691                                   Treatment (b)                                                                        733                                                                              1251                                                                             414                                                                              946                                                                              1048                                                                             2130                                                                             538                                                                              948                                                                              1030                                                                             743                                                                              2381                                                                             636                                   Treatment (c)                                                                        733                                                                              1248                                                                             411                                                                              946                                                                              1052                                                                             2130                                                                             546                                                                              954                                                                              1031                                                                             751                                                                              2384                                                                             529                                   Treatment (d)                                                                        735                                                                              1250                                                                             411                                                                              946                                                                              1052                                                                             2130                                                                             551                                                                              959                                                                              1036                                                                             755                                                                              2378                                                                             722                                   __________________________________________________________________________

Stability on Coercive Forces of Magnetic Recording Media Example 2

Magnetic coating formulations were separately prepared using themagnetic powders of Magnetic Powder Nos. (2), (3), (17), (45), (47) and(54) and the magnetic powders obtained by subjecting the above magneticpowders to the treatments (a)-(c) in accordance with Example 1, andtheir corresponding Solvents 1-3 to produce magnetic recording media inorder to determine variations in coercive force of the resultingrecording media from those of the corresponding magnetic powders.Results are shown in Table 5.

Incidentally, the "Variation in coercive force of magnetic recordingmedium" in Table 5 is a quotient obtained by dividing the coercive forceof each magnetic recording medium by that of its corresponding untreatedmagnetic powder.

Comparative Example 2

Using Solvents 4 and 5, the variations in coercive force were determinedin the same manner as in Example 2. Results are shown in Table 5.

Moreover, using the conventional Co-substituted barium ferrite magneticpowder (C-1) prepared in Comparative Example 1 and containing Co in agreat amount, the variations in coercive force were determined in thesame manner as in Example 2 (Solvents 1-5 were used as solvents for thispurpose). Results are shown in Table 5.

It is understood from the results of Example 2 and Comparative Example 2that the coercive forces of the magnetic recording media produced byapplying the magnetic coating formulations making use of the magneticpowders according to this invention and their corresponding solventscontaining at least 15 wt. % of at least one ketone compound aresubstantially the same coercive forces of their corresponding magneticpowders used and are very stable.

On the other hand, it is appreciated that when Solvents 4 and 5containing less than 15 wt. % of at least one ketone compound are used,the coercive forces of the resulting magnetic recording media areunstable.

It is further understood that when the Co-substituted barium ferritemagnetic powder prepared in Comparative Example 1 and containing Co in agreat amount is used, the coercive forces of the resulting magneticrecording media vary greatly if the kind of the solvent used and thewater content of the magnetic powder are changed, and are very unstable.

                                      TABLE 5                                     __________________________________________________________________________    Magnetic    Example 2                                                         powder No.  (2)                                                                              (3)                                                                              (17)                                                                              (45)                                                                             (47)                                                                             (54)                                                                             (2)                                                                              (3)                                                                              (17)                                                                              (2)                                                                              (2)                                                                              (45)                                                                             (47)                                                                             (54)                                                                             (45)                                                                              (45)              __________________________________________________________________________    Treatment of                                                                               *1                                                                               *1                                                                               *1  *1                                                                               *1                                                                               *1                                                                              (a)                                                                              (b)                                                                              (c) (b)                                                                              (c)                                                                              (a)                                                                              (b)                                                                              (c)                                                                              (b) (c)               magnetic powder                                                               Coercive force of                                                                         734                                                                              1250                                                                             1050                                                                              955                                                                              1033                                                                             748                                                                              731                                                                              1251                                                                             1052                                                                              733                                                                              733                                                                              951                                                                              1030                                                                             742                                                                              950 952               magnetic powder after                                                         the treatment, Hc (Oe)                                                        Solvent      1   2                                                                                3  1   2                                                                               3  2   3                                                                                1  3  1  2   3                                                                               1  3   1                Coercive force of                                                                         741                                                                              1252                                                                             1044                                                                              961                                                                              1042                                                                             755                                                                              735                                                                              1248                                                                             1058                                                                              736                                                                              738                                                                              960                                                                              1035                                                                             740                                                                              962 965               magnetic recording                                                            medium, Hc⊥ (Oe)                                                         Variation in coercive                                                                     1.01                                                                              1.00                                                                             0.99                                                                             1.01                                                                              1.01                                                                            1.01                                                                             1.00                                                                              1.00                                                                             1.01                                                                             1.00                                                                             1.01                                                                             1.01                                                                              1.00                                                                            0.99                                                                             1.01                                                                              1.01              force of magnetic                                                             recording medium,                                                             Hc⊥/Hc                                                                   __________________________________________________________________________    Magnetic    Comparative Example 2                                             powder No.  (2) (3)                                                                              (17)                                                                             (45)                                                                              (47)                                                                             (54)                                                                             (2) (45)                                                                             (C-1)                                                                            (C-1)                                                                             (C-1)                                                                            (C-1)                                                                            (C-1)                                                                             (C-1)                                                                            (C-1)              __________________________________________________________________________    Treatment of                                                                               *1  *1                                                                               *1                                                                               *1  *1                                                                               *1                                                                              (a) (a)                                                                               *1                                                                               *1  *1                                                                               *1                                                                               *1 (a)                                                                              (c)                magnetic powder                                                               Coercive force of                                                                         734 1250                                                                             1050                                                                              955                                                                              1033                                                                             748                                                                              731  951                                                                             581                                                                              581 581                                                                              581                                                                              581 691                                                                              529                magnetic powder after                                                         the treatment, Hc (Oe)                                                        Solvent      4    5                                                                                4                                                                               4    5                                                                               4  4    4                                                                               1  2   3  4  5   1  2                 Coercive force of                                                                         658 1107                                                                              924                                                                             1068                                                                              1244                                                                             861                                                                              610 1049                                                                             873                                                                              885 742                                                                              792                                                                              498 980                                                                              939                magnetic recording                                                            medium, Hc⊥ (Oe)                                                         Variation in coercive                                                                     0.90                                                                               0.89                                                                             0.88                                                                             1.12                                                                              1.20                                                                            1.15                                                                             0.83                                                                               1.10                                                                            1.50                                                                             1.52                                                                              1.28                                                                             1.36                                                                             0.86                                                                              1.69                                                                             1.62               force of magnetic                                                             recording medium,                                                             Hc⊥/Hc                                                                   __________________________________________________________________________     *1: Not treated.                                                         

Example 3

Most of the magnetic powders shown in Table 3 and their correspondingSolvents 1, 2 and 3 were used to produce magnetic recording media inorder to measure their coercive forces. Results are shown in Table 6.

Comparative Example 3

Some of the magnetic powders shown in Table 3 and their correspondingSolvents 4 and 5 were used to produce magnetic recording media in orderto measure their coercive forces. Results are shown in Table 7.

It is understood from the results of Tables 6 and 7 that the magneticrecording media obtained in accordance with the method of this inventionhave excellent stability on coercive force.

                  TABLE 6                                                         ______________________________________                                                         Coercive force                                                                             Variation in coercive                           Magnetic         of magnetic  force of magnetic                               powder           recording medium                                                                           recording medium                                No.    Solvent   Hc⊥ (Oe)                                                                              Hc⊥/Hc                                     ______________________________________                                         (1)   1         598          1.00                                             (4)   1         744          0.99                                             (5)   2         1455         1.01                                             (6)   1         738          1.00                                             (7)   3         1680         1.00                                             (8)   1         410          0.99                                             (9)   1         1520         0.99                                            (10)   3         893          1.01                                            (11)   1         950          1.00                                            (12)   1         2760         1.00                                            (13)   1         654          1.01                                            (14)   2         2550         1.00                                            (15)   2         857          1.01                                            (16)   1         706          0.99                                            (18)   1         754          1.00                                            (19)   1         309          0.99                                            (20)   3         355          0.99                                            (21)   1         720          0.99                                            (22)   2         845          1.01                                            (23)   1         700          1.01                                            (24)   1         1214         0.99                                            (25)   1         947          1.00                                            (26)   3         1688         1.00                                            (27)   2         601          1.01                                            (28)   1         2142         1.00                                            (29)   1         1427         1.00                                            (30)   1         853          0.99                                            (31)   1         572          1.02                                            (32)   1         361          1.02                                            (33)   1         490          1.00                                            (34)   2         671          0.99                                            (35)   1         863          1.00                                            (36)   1         2132         1.00                                            (37)   3         1525         0.99                                            (38)   1         984          1.00                                            (39)   1         652          0.99                                            (40)   3         2801         1.00                                            (41)   1         730          1.00                                            (42)   1         841          1.01                                            (43)   1         540          1.00                                            (44)   1         626          1.01                                            (46)   2         727          1.02                                            (48)   3         725          1.02                                            (49)   3         851          1.02                                            (50)   1         310          1.00                                            (51)   2         1142         1.01                                            (52)   1         445          0.99                                            (53)   2         1564         1.00                                            (55)   3         950          0.99                                            (56)   3         2383         1.00                                            (57)   1         2536         1.00                                            (58)   1         844          1.00                                            (59)   2         921          0.98                                            (60)   1         642          0.99                                            (61)   2         1958         1.01                                            (62)   2         459          0.99                                            (63)   3         713          0.99                                            (64)   3         735          0.99                                            (65)   1         562          1.00                                            (66)   3         764          1.02                                            (67)   3         961          0.99                                            (68)   1         1350         1.00                                            (69)   2         1243         0.99                                            ______________________________________                                    

                  TABLE 7                                                         ______________________________________                                                         Coercive force                                                                             Variation in coercive                           Magnetic         of magnetic  force of magnetic                               powder           recording medium                                                                           recording medium                                No.    Solvent   Hc⊥ (Oe)                                                                              Hc⊥/Hc                                     ______________________________________                                         (4)   4         845          1.12                                             (6)   4         618          0.83                                             (8)   5         359          0.87                                            (10)   5         1134         1.29                                            (11)   4         1060         1.12                                            (13)   4         830          1.28                                            (18)   5         611          0.81                                            (27)   4         896          1.50                                            (29)   5         1106         0.77                                            (30)   4         733          0.85                                            (43)   4         596          1.10                                            (48)   4         623          0.88                                            (50)   5         458          1.47                                            (52)   5         511          1.14                                            (56)   4         2792         1.17                                            (60)   5         562          0.87                                            (62)   4         637          1.38                                            (63)   5         643          0.89                                            ______________________________________                                    

What is claimed is:
 1. A method of producing a magnetic recording mediumin which a magnetic coating formulation is prepared by dispersing andkneading a magnetic powder together with a dispersant, solvent, binderand one or more other additives, wherein said magnetic coatingformulation containing a hexagonal ferrite magnetic powder is applied tothe surface of a substrate and dried, characterized in that thehexagonal ferrite magnetic powder is represented by the followinggeneral composition formula (I) and has an average particle diameter ofat most 0.1 μm, a ratio of the maximum diameter to the maximum thickness(a plate ratio) of at most 15, a particle size distribution (σgd) of atmost 3.4 in terms of the geometric standard deviation based on thenumber and a coercive force of 200-3,000 Oe, and a solvent containing atleast 25 wt % of at least one ketone compound having at most 10 carbonatoms is used in the magnetic coating formulation

    (Fe).sub.a (Co).sub.b (M.sup.1).sub.c (M.sup.2).sub.d (M.sup.3).sub.e (O).sub.f                                                 [I]

wherein M¹ means at least one metal element selected from Ba, Sr, Ca andPb, M² denotes at least one metal element selected from Ti, Zr, Hf, Si,Ge, Sn, Mn, Mo, W, V, Ce, Nd, Sm, B and Nb, M³ stands for at least onemetal element selected from Mg, Ni, Cr, Cu, Zn, Cd, In, Ga, Bi, La, Y,P, Sb and Al, a, b, c, d, e and f respectively represent the numbers ofFe, Co, M¹, M², M³ and O atoms, a is a number of 8.0 to 12.0, b is anumber of 0 to 0.5, a ratio of b to a is at most 0.055, c, is a numberof 0.3 to 6.0, d and e are individually a number of 0 to 6.0 with theproviso that when b is a number other than 0 and M² is Ti alone, e is anumber other than o, d+e is a number of 0.1 to 8.0, and f is the numberof oxygen atoms satisfying the atomic valences of the other elements. 2.The method of claim 1, wherein the hexagonal ferrite magnetic powdercontains the individual components in such proportions that a is 8.0 to12.0, b is a number of 0 to 0.4, a ratio of b to a is at most 0.05, c isa number of 0.3 to 4.0, d is a number of 0 to 4.0, e is a number of 0 to4.0 with the proviso that when b is a number other than 0 and M² is Tialone, e is a number other than 0, d +e is a number of 0.2 to 5.0, and fis the number of oxygen atoms satisfying the atomic valences of theother elements in the general composition formula [I].
 3. The method ofclaim 1, wherein the hexagonal ferrite magnetic powder has a ratio ofthe maximum diameter to the maximum thickness (a plate ratio) of at most10.
 4. The method of claim 1, wherein the hexagonal ferrite magneticpowder has a particle size distribution (σgd) of at most 3.1.
 5. Themethod of claim 1, wherein the hexagonal ferrite magnetic powder isprepared in accordance with the coprecipitation-flux method.
 6. Themethod of claim 1, wherein the solvent used in the magnetic coatingformulation contains 15-90 wt. % of at least one ketone compound havingat most 10 carbon atoms.
 7. The method of claim 1, wherein said at leastone ketone compound having at most 10 carbon atoms is selected fromacetone, methyl ethyl ketone, methyl isobutyl ketone, cyclohexanone,methyl n-propyl ketone, diethyl ketone, hexanone-2, hexanone-3,di-n-propyl ketone, diisopropyl ketone, diisobutyl ketone, di-n-amylketone, acetylacetone, prone, cyclopentanone and cycloheptanone.